Bone cement is commonly used to attach a prosthetic implant such as a hip stem to living bone tissue such as a prepared intramedullary canal of a femur. Bone cements are typically acrylic polymeric materials which undergo polymerization when thoroughly mixed together. The preferred conventional bone cement is composed of a powder, polymethyl methacrylate (PMMA) polymer and a liquid, methyl methacrylate (MMA) monomer. The polymerization requires an initiator to catalyze hardening. The initiator is typically a tertiary amine such as aniline or toluidine. The amine initiator cleaves the benzoyl peroxide in the PMMA powder component to produce the free radicals which initiate the bonding of the liquid monomer.
In a surgical application, the bone cement may be contaminated by blood, body fluids, water, and air bubbles which weaken the interlock between the bone tissue and implant. Prosthetic implants have been developed which include an outer layer of cement molded to the implant. By molding a layer of cement to the implant in a controlled environment, the interlock between the implant and cement is greatly improved. During the implant procedure, an additional amount of bone cement is used to secure the implant to the bone tissue. The additional bone cement interacts molecularly with the layer of cement molded to the implant and interdigitates with the bone tissue to provide a positive interlock.
Molding or applying bone cement to metal or composite prosthetic implants presents a variety of production difficulties. The bone cement must be applied to the implant to reduce the number of contaminants and voids as much as possible to ensure a positive interlock between the implant and cement. In an a preferred manufacturing setting, the cement is applied by a molding process. The implant is placed in a mold and the cement is mixed and extruded into the mold where it is polymerized around the implant. Extruding polymerizing bone cement into a mold, however, is problematic. The viscosity of acrylic cements increases with the time elapsed after mixing the monomer and polymer components. In an extrusion process, the bone cement, after mixture, quickly becomes too stiff to be easily extruded into the molds. In addition, the amine initiators in the cement which initiate polymerization discolor during sterilization, turning yellow or brown in color.